1. Field of the Invention
The present invention relates to a novel acoustic wave sensor for sensing acoustic waves in a fluid typically air. The invention employs optical techniques, and more particularly depends upon the phase modulation which occurs when coherent light is passed through a fluid in which acoustic waves occur. The phase modulation may be imposed on a carrier provided by optical heterodyning and then detected electrically. The electrical signal recovered in this manner is representative of the acoustic waves.
2. Description of the Prior Art
In the patent application of Gordon Jacobs, Ser. No. 507,528 filed Sep. 19, 1974 entitled “Laser Hydrophone and Virtual Array of Laser Hydrophones”, an acoustic sensor employing optical techniques was earlier proposed. The sensor, which was termed a “hydrophone”, since it was designed for use in water, employed a laser beam which was focused on a small “focal” volume of water in which natural light scattering matter was suspended. The scattering matter, which vibrates in synchronism with any acoustic waves present, produces a phase modulation of the scattered light. The phase modulation was then recovered by optical heterodyne and sensitive phase demodulation techniques. The Jacobs arrangement contemplated both single hydrophones and arrays of hydrophones. In general, optical arrays, such as the Jacobs arrays, produce less hydrodynamic disturbance than the known large area piezoelectric arrays.
The Jacob's arrangement was dependent upon a “doppler” type shift in frequency or phase, the doppler shift being caused by particle motion, toward and away from the sensing beam. Thus the maximum sensitivity was obtained by pointing the laser beam in a direction toward the source of acoustic waves and perpendicular to the wave fronts.
The present invention shares certain of the objectives and in using optical techniques, certain of the means of the foregoing Jacobs' invention.
The Jacobs invention is unlike the present invention which has application to the detection of acoustic waves in the air. The present invention has application to security systems, as for instance in the monitoring of sounds occurring along the perimeter of a property.